1. Field of the Invention
The present invention relates to a high-pressure fuel pump assembly mainly for use in a cylinder-injected engine, etc.
2. Description of the Related Art
Engines in which fuel is injected into the engine cylinder, the so-called "cylinder-injected" or "direct injection engines", are known among both diesel engines and gasoline engines. Cylinder-injected engines of this kind demand that sufficiently high fuel injection pressure be provided and also demand that fuel pressure surges be minimized to ensure stable injection. To these ends, compact single-cylinder high-pressure fuel pumps have been proposed which are of simple construction and inexpensive to manufacture.
FIG. 12 is a block diagram showing the layout of a high-pressure fuel pump assembly 100 to which the present invention can be applied. In FIG. 12, a delivery pipe 1a supplies fuel to fuel injection valves 1, which inject fuel into each of the engine cylinders (not shown). This fuel is supplied to the high-pressure fuel pump assembly 100 through a low-pressure fuel supply passage 5 from a fuel tank (not shown) by means of a low-pressure fuel pump (not shown). The main component parts of the high-pressure fuel pump assembly 100 include: a low-pressure damper 13; a high-pressure fuel pump 20; a high-pressure damper 60; and a check valve 70. The high-pressure fuel pump 20 comprises: a reed valve assembly 30 having an intake valve 22 disposed in a fuel inlet 5a and a discharge valve 21 disposed in a fuel outlet 4a; and a high-pressure fuel pump main body portion 23. The check valve 70 opens when the pressure of the fuel on the fuel supply port 9 side, which connects to the delivery pipe 1a of the high-pressure fuel pump assembly 100, is lower than the pressure of the fuel on the high-pressure damper 60 side.
Fuel pressure surges in the fuel which is supplied to the high-pressure fuel pump assembly 100 through the low-pressure fuel supply passage 5 are absorbed by the low-pressure damper 13, the fuel is pressurized by the high-pressure fuel pump assembly 100, surges in the pressurized fuel are absorbed by the high-pressure damper 60, and the fuel passes through the check valve 70 and is supplied to the delivery pipe 1a from the fuel supply port 9. A passage 10 connecting to a high-pressure regulator (not shown) is disposed between the fuel supply port 9 and the delivery pipe 1a.
A cross-section of the construction of the high-pressure fuel pump assembly 100 is shown in FIG. 13. An enlarged cross-section of the region surrounded by the dot-and-dash line in FIG. 13 is shown in FIG. 14.
In FIG. 13, a cylindrical recessed portion 40a is formed in the casing 40 of the high-pressure fuel pump assembly 100. A high pressure fuel pump 20, which comprises a reed valve assembly 30 and a high-pressure fuel pump main body portion 23, is disposed in the recessed portion 40a.
The high-pressure fuel pump 20 is constructed by arranging the reed valve assembly 30 and the high-pressure fuel pump main body portion 23 one on top of the other from the bottom portion 40b of the casing 40.
Details of the reed valve assembly 30 in the high-pressure fuel pump 20 are shown in FIG. 14.
The reed valve assembly 30 comprises two plates 31, 33 and a thin valve plate 32 sandwiched between the two plates 31, 33. The plate 31 side of the reed valve assembly 30 is disposed in contact with the bottom portion 40b, and two adjoining passages are formed in each of the two plates 31, 33 to allow fuel to pass through. Two of the passages in the plates 31, 33 have larger cross-sections than their adjoining counterpart passages so that the valves in the valve plate 32, namely the intake valve body 32a and the discharge valve body 32b, each operate in one direction only as shown by the broken lines in the figure. The adjoining counterpart passages respectively form a fuel inlet 5a, which stops the backward motion of the intake valve body 32a and supplies fuel to the high-pressure fuel pump 20, and a fuel outlet 4a, which stops the backward motion of the discharge valve 32b and supplies fuel to the fuel discharge passage 4 from the high-pressure fuel pump main body portion 23.
The high-pressure fuel pump main body portion 23 is disposed in contact with the reed valve assembly 30.
A sleeve 41 and a fuel pressurizing chamber 45, which is surrounded by a piston 43 slidably inserted into the sleeve 41, are formed in the high-pressure fuel pump main body portion 23.
Cylindrical recesses are formed in both ends of the piston 43. A coil-shaped spring 36, which pushes the piston 43 downwards in the direction which expands the fuel pressurizing chamber 45, is disposed in a compressed condition between a spring holder 37 and the piston 43 in the recess in the reed valve assembly 30 end of the piston 43 to draw fuel in. A tappet 46 is secured in the recess in the other end of the piston 43 so as to be able to rotate freely. The tappet 46 is in contact with a cam 48, which drives the high-pressure fuel pump. The cam 48 is part of a camshaft of an engine (not shown), or is disposed on the same axis thereto, and the camshaft moves together with a crankshaft of the engine to complete one revolution for every two revolutions of the crankshaft, the piston 43 reciprocating according to the profile of the cam 48. The volume of the fuel pressurizing chamber 45 is changed by the reciprocation of the piston 43, and pressurized fuel is discharged to the fuel discharge passage 4.
A drainage chamber 52, which holds fuel which leaks out from the fuel pressurizing chamber 45 through the sliding portion 51 between the sleeve 41 and the piston 43, is formed between the sleeve 41 and a housing 42. The fuel which leaks out into the drainage chamber 52 is returned to the fuel tank (not shown) by means of a drainage passage 8 and a check valve 11, which is shown in FIG. 12. A metal bellows 53, which follows the reciprocation of the piston 43 and seals in the fuel which leaks out into the drainage chamber 52, is secured by welding to the end of the housing 42. The other end of the bellows 53 is welded to a cap 54, which is airtightly secured to the piston.
The reed valve assembly 30 and sleeve 41 are fastened to the cylindrical recessed portion 40a of the casing 40 by a threaded bush 35 by means of the housing 42. A seal is formed between the casing 40 and the housing 42 by means of an O-ring 55 to prevent fuel from leaking outside. A bracket 57 is disposed on the outside of the housing 42 and is sealed by an O-ring 56.
A recessed portion 40c is formed in the housing 40. A high-pressure damper 60 is fastened into this recessed portion 40c. High-pressure gas is enclosed in a space in the high-pressure damper 60, which is sealed by a thick substantially-cylindrical case 61 and a thin disk-shaped metal diaphragm 62. The metal diaphragm 62 moves to equalize the pressure of the high-pressure gas and the pressure of the fuel which flows from the fuel discharge passage 4 into a damper chamber 64, which is surrounded by the metal diaphragm 62 and a plate 63. The volume of the damper chamber 64 is thereby changed, absorbing pressure surges in the fuel in the fuel discharge passage 4.
A check valve 70, which opens when the pressure in the fuel on the delivery pipe 1a side is lower than the pressure of the fuel on the high-pressure fuel pump assembly side, is disposed in the fuel discharge passage 4 between the high-pressure damper 60 and the fuel supply port 9. The check valve 70 is provided to maintain the fuel within the delivery pipe 1a at high pressure even when the engine is stopped and to improve the starting of the engine.
The check valve 70 comprises: a plate 71; a housing 72; a spring 73; an O-ring 74; a spherical valve body 75; and a valve seat 76. The valve seat 76 has a tapered portion in the end of a cylindrical opening, which is a fuel passage, and the valve body 75, which is pressed by a coil spring 73, seals this tapered portion, closing the fuel discharge passage 4. The spring 73 is positioned by means of the housing 72 by engaging and fastening the thread on plate 71 in the thread in the casing 40, and imparts a fixed spring load to the valve body 75. The O-ring 74 is disposed between the casing 40 and housing 72 to prevent fuel from leaking outside.
During the discharge stroke, the discharge valve body 32b in the reed valve assembly 30 opens and the high-pressure pump 20 discharges fuel, then the high-pressure pump 20 enters its intake stroke and the pressure in the fuel pressurizing chamber 45 decreases while the intake valve body 32a is still open. At this time, a back flow of fuel occurs due to the difference in pressure between the high-pressure fuel on the high-pressure damper 60 side of the discharge valve 21 and the fuel on the depressurized fuel pressurizing chamber 45 side. The greater the volume of the portion between the discharge valve 21 and the check valve 70 which is filled with fuel, that is to say, the greater the combined volume of the high-pressure damper 60 and the fuel discharge passage 4, the smaller the decrease in fuel pressure on the high-pressure damper 60 side of the discharge valve 21 due to back flow, that is to say, the greater the difference between the fuel pressure on the high-pressure damper 60 side of the discharge valve 21 and the fuel pressure on the depressurized fuel pressurizing chamber 45 side, and the amount of back flow therefore increases, reducing the discharge flow efficiency (volumetric efficiency).
This reduction in discharge flow efficiency is particularly noticeable when a high fuel temperature is required to keep the discharge pressure high and when the discharge pressure is raised because the viscosity of the fuel decreases. Also, if the cross-sectional area of the fuel discharge passage 4 is small, the flow of the fuel is choked and the fuel cannot flow through the passage sufficiently, and therefore the loss of pressure is great and the maximum pressure in the high-pressure pump 20 is increased, further reducing the discharge flow efficiency. In addition, when the discharge pressure of the high-pressure pump 20 is increased in this way, the load on the cam 48 which drives the high-pressure pump is also increased, increasing the amount of friction at the surface where the cam 48 is in contact with the tappet 46. Furthermore, if the discharge pressure of the high-pressure pump 20 is increased, the amount of fuel which leaks into the drainage chamber 52 from the sliding portion 51 between the sleeve 41 and the piston 43 also increases and the flow of fuel is poor where the cross-sectional area of the passage between the sleeve 41 and the housing 42 is small, giving rise to surges in pressure within the metal bellows 53 as the piston 43 reciprocates, reducing the durability of the metal bellows 53.